This invention relates to an improved process for drying gases such as natural gas by treatment of such gas with a glycol drying agent. More particularly, this invention relates to improvements in the methods for reconcentrating glycol drying agents which have been used to remove water from natural gas streams.
When gases, more particularly natural gas, are dried with a glycol, more particularly triethylene glycol, water vapor is absorbed from the gas and a water-containing glycol is formed. If this glycol is to be reused for gas-drying purposes, the water-containing glycol must be reconcentrated to remove most of the water therefrom. Accordingly, the water-containing glycol is generally heated and the major part of the water is thereby vaporized and separated out, thereby forming a concentrated glycol of low water content. This reconcentrated glycol is returned into the gas-drying stage. Mere heating of the moist glycol cannot be used to accomplish complete drying of the glycol, since at maximum heating temperatures for glycols (about 205.degree. C. with triethylene glycol), a drying of only up to about 99.0 percent glycol is realized. When operating continuously, higher heating temperatures lead to a partial thermal decomposition of the triethylene glycol, so that the advantage of the higher reconcentration is lost due to increased expense for the replacement of the glycol. Efforts have been made, however, to increase the concentration of the glycol still further, since the residual moisture of the gas dried with glycol proportionately decreases as the concentration of the glycol used for the drying is increased. It is thus possible, for example, when using a 99.9 percent triethylene glycol (instead of one which is only 99 percent) and a contact temperature with the gas to be dried of 20.degree. C., to reduce the dew point of the gas from about -20.degree. C. to about -40.degree. C.
It is also known to cool natural gas with a content of liquid hydrocarbons in the dried or undried state down to temperatures of, for example, -30.degree. to 0.degree. C. and thereby to condense and separate out the liquid hydrocarbons and the moisture possibly contained in the gas. With undried natural gas, dried glycol can be sprayed as a hydrate inhibitor into the stream of natural gas prior to cooling and moist glycol and liquid hydrocarbon are separated out as condensate after the expansion of the natural gas to pipeline pressure. This moist glycol, like the moist glycol which is formed from drying of the natural gas by absorption with glycol, is again reconcentrated and is used for spraying or for absorption purposes.
It is also known to improve still further the degree of drying of the thermally reconcentrated glycol by stripping the glycol being reconcentrated with a heated dry gas. Serving as stripping gas is dried natural gas, which is heated in the reboiler and is then brought into contact with the preconcentrated glycol discharging from the reboiler (Cf. U.S. Pat. No. 3,105,748). The disadvantage of this process consists in that a part of the valuable product gas in consumed for stripping purposes. The stripping gas charged with the water vapor is generally blown off into the atmosphere or burnt. To avoid this, the stripping gas can also be recirculated. In this case, the stripping gas is cooled and an aqueous phase is deposited in a separator. The stripping gas as thus dried returns, together with supplementary gas, into the stripping section of the reconcentration plant (Cf. U.S. Pat. No. 3,867,112). It is true that the consumption of stripping gas and the contamination of the air are smaller in this case, but the expense for the working up and recirculation of the stripping gas is considerable.
Finally, it is also known to carry out the absorption treatment of a moist, hydrocarbon-containing natural gas with a mixture of glycol and liquid hydrocarbon. The moist glycol and the liquid hydrocarbon phase charged with volatile hydrocarbons are separated after being heated and the hydrocarbon phase is combined with the hot glycol in the stabilizer section of the regeneration plant. The volatile hydrocarbons are thereby driven off from the liquid hydrocarbon phase and cause a further concentration of the pre-concentrated glycol, while the stabilized hydrocarbon liquid returns together with the dried glycol into the absorption stage (Cf. U.S. Pat. No. 3,321,890). With this operating procedure, it is necessary to have the joint circulation of glycol and hydrocarbon liquid between the absorption stage and the regeneration stage, and this is a procedure which generally causes the glycol to reach a foaming condition during the regeneration.
This invention has for its object provision of a new process for the thermal reconcentration of glycol, using stripping gas, with which the disadvantages of the former processes operating with the use of stripping gas are avoided. In other words, the use of dried natural gas as stripping gas and also the working up and recirculation of stripping gas and the recirculation of hydrocarbon between absorption stage and regeneration stage can be avoided.